Until recently, there has been little information about why one in three Americans will develop cancer during their lifetime. It has been difficult to distinguish genetic factors from shared carcinogenic exposures. Our goal has been to identify specific susceptibilities that initiate tumor formation which can be transmitted as genetic traits. Our work on retinoblastoma led to the cloning of the first tumor suppressor gene and confirmation of Knudson's theory that a mutant gene termed a, "tumor suppressor gene", could be carried in the germ line. This work showed that a germ line mutation in a tumor suppressor gene could provide a dramatic susceptibility to specific pediatric malignancies. In an effort to determine if tumor suppressor genes might provide susceptibility to some of the common adult malignancies, we have studied rare families with the "Li-Fraumeni Syndrome". These families get breast cancer and a diverse set of pediatric malignancies. We have shown that affected members in the families with Li-Fraumeni Syndrome carry germ line mutations in a known tumor suppressor gene, p53. Our aim is to determine the fraction of Li-Fraumeni syndrome families that has these germ line p53 mutations, and the characteristic of the cancer patients outside of these rare families who also carry germ line p53 mutations so that they can be properly followed and treated. We have found germ line p53 mutations in women with breast cancer who are not members of the Li-Fraumeni syndrome families, and found that de novo germ line p53 mutations can be detected in children who develop second primary cancers. We propose to examine the frequency of germ line p53 mutations in women with various forms of breast cancer, and to identify children who are at risk for developing second malignant neoplasms by screening for germ line p53 mutations. The types of mutations carried in the germ line appear to represent a specific subset of p53 mutations. Structural studies suggest that the p53 mutations tolerated in the germ line may be limited to mutations which have undergone only minimal structural perturbations. The structural and functional analysis of the germ line p53 mutations provide a unique opportunity to probe the normal function of this important gene. The information gained from these studies of individuals with germ line p53 mutations and the structural and functional analysis of these mutations should eventually lead not only to strategies for screening, but also to improvements in cancer control.